Cutting inserts or tools having an elongated form with a generally polygonal cross section and cutting edges extending generally longitudinally from one end of the cutting insert to the other are disclosed in U.S. Pat. Nos. 147,041; 3,188,717 and 3,421,196. A turning and planing tool is described in the '041 patent. At each corner of its cross section the tool has a cutting edge which runs along the length of the tool. The cross section is obtained by arranging the clearance angle and the cutting angle in such a way that they succeed each other all around the cross section of the tool. The '196 patent describes a disposable cutting insert in the form of a generally rectangular block having at least four parallel cutting edges. At least one of the cutting edges is chamfered to form a relatively blunt cutting edge. The chamfered cutting edge is less subject to shock damage than the unchamfered edges. During a cutting operation the insert can be indexed to present the chamfered cutting edge when stock is to be removed from a workpiece portion having an irregular surface.
U.S. Pat. No. 3,188,717 discloses a cutting bit which is basically a parallelepiped having front and rear generally square faces and four rectangular or square side faces disposed normal to each other and forming cutting edges at their respective intersections. Each of the side faces bounded by a pair of cutting edges is provided with a pair of longitudinally extending depressed chip guides. FIGS. 6 and 7 of the '717 patent show a portion of a milling cutter carrying a plurality of such cutting bits. However, the cutting bits do not possess secondary cutting edges, termed wipers, for achieving a smoothly finished surface in a single pass.
U.S. Pat. No. 3,955,259 discloses an indexable cutting insert for a milling cutter having four main cutting edges and four associated secondary cutting edges. The purpose of a secondary cutting edge is to make a finishing cut. The cutting insert has the form of a polygonal flat plate and each main cutting edge meets its associated secondary cutting edge at one end thereof at a cutting tip (or cutting corner). All in all there are four cutting tips.
It would be advantageous, from the point of view of the strength of the cutting insert, if the cutting insert of the '259 patent were to be designed to have a substantially square cross section, instead of a rectangular (flat plate) cross section. It would be further advantageous, if each main cutting edge would have an associated secondary cutting edge at each end thereof, giving rise to eight cutting tips. When performing milling operations which utilize approximately half the length of a main cutting edge, such a cutting insert with eight cutting tips would have eight effective main cutting edges. However, associated with each secondary cutting edge, of the cutting insert of the '259 patent, is a pair of bevel faces, and at a given end portion of the cutting insert one pair of bevel faces, associated with one corner, is separated from the other pair of bevel faces, associated with the other corner of that end portion, by a flat surface which extends across the cutting insert between the top and bottom surfaces. This particular structure of the end portions of the cutting insert of the '259 patent does not facilitate the extension of the cutting geometry to eight cutting tips.
It is an object of the present invention to provide a cutting insert that has a substantially square cross section with four rectangular or square side faces, forming main cutting edges at their respective intersections, wherein each main cutting edge is provided with a secondary cutting edge (wiper) for making a finishing cut, at each end thereof, giving rise to eight cutting corners.